Cryogenic Q-band (35Â GHz) probehead featuring large excitation microwave fields for pulse and continuous wave electron paramagnetic resonance spectroscopy: Performance and applications

The construction and performance of a Q-band (35 GHz) cryogenic probehead for pulse electron paramagnetic resonance and continuous wave electron paramagnetic resonance measurements with down-scaled loop gap resonators (LGRs) is presented. The advantage of the LGR in comparison to TE012 resonators lies in the large B1 microwave (mw) fields that can be generated with moderate input mw power. We demonstrated with several examples that this allows optimal performance for double-quantum electron coherence, HYSCORE, and hyperfine decoupling experiments employing matched and high turning angle mw pulses with high B1-fields. It is also demonstrated that with very low excitation power (i.e. 10-40 mW), B1-fields in LGRs are still sufficient to allow short mw pulses and thus experiments such as HYSCORE with high-spin systems to be performed with good sensitivity. A sensitivity factor Λrs of LGRs with different diameters and lengths is introduced in order to compare the sensitivity of different resonant structures. The electromagnetic field distribution, the B1-field homogeneity, the E1-field strength, and the microwave coupling between wave guide and LGRs are investigated by electromagnetic field calculations. The advantage and application range using LGRs for small sample diameters is discussed.